1. Field of the Invention
The present invention relates to techniques for superplastic forming of parts, and more particularly, for control of thinout in such parts.
2. Background of the Invention
Superplastic forming hereinafter (SPF) is a metal forming process used throughout the aerospace industry for manufacturing detailed parts and built-up structures. The design flexibility that is offered by SPF has resulted in substantial cost savings in the fabrication of detail parts and assemblies. Further savings have been apparent in the reduction of weight in aircraft. The prior art SPF process for manufacturing parts consists of several steps. These steps are illustrated in FIGS. 1A to 1D and can be summarized as follows: heating a die to an appropriate temperature for a particular metal alloy; placing a metal sheet, also referred to as a blank, in the die; closing a lid to the die; applying restraining forces to hold the die and lid together; applying a forming gas pressure to the blank in order to push the blank into the die cavity; completing the time required in the forming cycle; and removing the finished part from the die.
FIG. 2 shows a schematic plan view of the die with the lid removed for illustration purposes. The blank or sheet 10 is supported in the regions 12 surrounding the sealed area 14 by the lower die 16, as shown in FIG. 1A. The double lines 18 outline the seal area, within which a part will be formed. The reason that the material does not thinout uniformly is that once the lid is closed on the SPF die, the periphery of the material is restrained such that the material is not allowed to "draw-in" the material outside of the seal area.
FIG. 3 shows a schematic cross section view of a part formed by SPF. The dotted lines show where the part will be cut or trimmed. The run-out is in the die region outside of the net part area. A correctly designed die will optimize the run-out configuration so that thinout is minimized in the part area and maximized in the run-out material.
FIG. 4 is a side elevation cross-section illustrating the thinout problem. For example, the part thicknesses at 20 and 22 are very thin, and could potentially be below the thicknesses specified by the Engineering drawing.